1. Field of the Invention
The present invention relates to more efficiently and effectively imparting a glossy and smooth surface to a paper web. More particularly, the present invention relates to a process using shear to impart desirable characteristics to a paper web.
2. Description of the Related Art
Compression of paper webs has been used to impart various different characteristics. These include imparting a glossy finish to a paper web or reducing the pore size of a web, such as an aramid paper web, in conjunction with heat.
Paper webs having a glossy and smooth finish are occasionally required or desired. Glazers are available and are known in the art. Such equipment does impart a glossy and smooth finish to paper webs by compressing the paper between two different curved surfaces. A combination of hard and soft rolls can also be used to provide a glossy finish. Problems occur, however, if the soft roll is damaged such that its surface is nicked or wrinkled, which can easily occur. The resulting paper is therefore damaged, which is a major problem. Being able to make a paper web having a glossy and smooth finish employing more conventional equipment, e.g., calender rolls of the same material, would be desirable. Using conventional steel calender rolls would be particularly advantageous.
Aromatic polyamide (aramid) paper is made on a fourdrenier paper machine from 0.25 inch long by 2 denier aramid fibers and aramid fibrid. The fibrid is a small irregularly shaped piece of aramid polymer that is much larger in two dimensions than it is in the third dimension. It is like a microscopic corn flake in shape. The large dimensions are on the order of 5 to 25 micrometers while the third and smaller dimension is about 0.01 to 1 micrometers. The fibrid serves as the bonding agent for the fibers. The paper made on the paper machine is surprisingly strong. However, the pore structure of the paper prevents it from reaching its maximum utility as electrical insulation. For many electrical insulation applications it is necessary to reduce the pore structure of the paper. This is done commercially by heating the paper to about 350° C. and then running the sheet through a two steel roll nip. This increases the density of the paper and reduces the pore structure. Controlled heating of the paper is difficult, and there is some thermal degradation of the fiber structure.
More specifically, the aramid paper is heated to a very high temperature so that the polymer is softened. While in this state the sheet is compressed by running it through a two steel roll nip. Pores are reduced in size by this densification. However, the structure of the sheet is unchanged. Because of this the sheet recovers some of its original shape after passing through the nip. As the sheet recovers part of its original shape there is some rebound in caliper and some increase in pore structure. Even after such a harsh step, the basic pore characteristics of the paper remain, although the pore size is much smaller due to the compression.
Accordingly, one object of the present invention is to provide a process to more effectively and efficiently impart desirable characteristics to a paper web.
Another object of the present invention is to provide a novel process for glazing a paper web.
Still another object of the present invention is to provide a process which more easily and effectively reduces the pore structure of aramid papers.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.